1. Field of the Invention
The present invention relates to a junction termination structure of a semiconductor device such as a diode, a MOSFET, an IGBT, a thyristor, and a transistor.
2. Related Background Art
FIG. 11 is a plane diagram showing the structure of a related semiconductor device, taking a power diode having high-breakdown voltage as an example. FIG. 12 is a cross sectional diagram taken along the A-A′ line in the plane diagram in FIG. 11. As is seen in FIG. 11 and FIG. 12, this semiconductor device includes an N-type base region 1, a P-type base region 2, an N-type cathode region 3, an anode electrode 4, a cathode electrode 5, P-type ring regions 8, and an N-type stopper region 6. A stopper electrode 7 is further formed on a major surface of the stopper region 6. An insulation film 10 is formed on a major surface of the base region 1 between the stopper electrode 7 and the anode electrode 4. This insulation film 10 is not formed on the ring regions 8, but gaps are formed on the ring regions 8, and ring electrodes 9 are formed to fill these gaps.
The N-type base region 1, the P-type base region 2, the N-type cathode region 3, the anode electrode 4, and the cathode electrode 5 constitute a diode portion, and the P-type ring regions 8 and the N-type stopper region 6 constitute a junction termination relaxing portion. Note that FIG. 11 shows the plane diagram in which the anode electrode 4, the ring electrode 9, the insulation film 10, and the stopper electrode 7 are omitted.
Further, as a semiconductor device of a high-breakdown-voltage planar P-i-N diode, the structure shown in FIG. 21 is well known. As shown in FIG. 21, the semiconductor device includes an N− base region 101, an N+ cathode region 102, a P anode region 103, a guard ring 104, field limiting rings 105, a channel stopper 106, a cathode electrode 107, and an anode electrode 108. Further, an insulation film 211 is formed on a termination region 110 surrounding the guard ring 104, and an electrode 109 is formed on the channel stopper 106.
When a voltage is applied between the anode electrode 108 and the cathode electrode 107 so as to reverse-bias a pn-junction formed by the N− base region 101 and the P anode region 103, a depletion region expands in the N− base region 101 from this pn-junction toward the cathode electrode 107. The depletion region also expands from the guard ring 104 at the same time, but when the depletion region reaches the field limiting ring 105a, a potential of the field limiting ring 105a is fixed at a value at this instant, and the depletion region starts to expand from the field limiting ring 105a. Thus, potentials of the field limiting rings 105 are subsequently fixed and the depletion region expands from the field limiting rings 105, thereby relaxing an electric field strength in an edge portion of the guard ring 104, so that a high breakdown voltage is obtainable. Therefore, with the increase in breakdown voltage, the number of the field limiting rings 105 needs to be increased.
FIG. 22 shows the electric field right under the insulation film 211 when the pn-junction formed by the N− base region 101 and the P anode region 103 is reverse-biased. The sum of areas of the portions indicated by hatching in FIG. 22 is a reverse bias voltage.
In the semiconductor device shown in FIG. 11 and FIG. 12, a high breakdown voltage is obtained by the optimum design (layout) of the P-type ring regions 8, but in this method, as an applied voltage becomes higher, the number of the P-type ring regions 8 also needs to be increased, which has posed a problem of difficulty in optimum design. Generally, the potentials of the P-type ring regions 8 are not fixed, and when a high voltage is applied, the design for uniformly dispersing the electric field, namely, the design of the number of the P-type ring regions 8 and the intervals therebetween is more difficult for a higher-breakdown-voltage product. For example, when 1000 V is applied to the stopper electrode 7 and the cathode electrode 5 and 0 V is applied to the anode electrode 4, this 1000 V voltage is divided among the P-type ring regions 8 in order to maintain the breakdown voltage between the stopper electrode 7 and the anode electrode 4, but it is difficult to specify a numerical value of the voltage of each of the P-type ring regions 8, which makes the design of the breakdown voltage extremely difficult.
In addition, even when appropriate design is achieved, there still remains such a problem that, if an interface state due to heavy metal contamination and so on is generated on an interface between the surface insulation film 10 and the N-type base region 1, the optimum conditions are not satisfied. Therefore, such a semiconductor device also confronts a problem of being susceptible to a disturbance in a fabrication process.
Further, the semiconductor device shown in FIG. 21 has such a problem that, as is seen from FIG. 22, the electric field is not generated in the field limiting rings 105 since they themselves are not depleted, which necessitates increasing the length of a termination region (termination length) in order to attain a predetermined breakdown voltage. Therefore, the semiconductor device shown in FIG. 21 has such a problem that, since the number of the field limiting rings 105 has to be increased in order to attain a high breakdown voltage, a termination length L becomes longer as the breakdown voltage becomes higher, so that the area of the P anode region 103 serving as an actual current path becomes smaller even with the same chip area, which worsens a on-state characteristic.